1. Field of the Invention
The present invention is directed to a method and device for detecting a variation, such as a variation in the concentration of a substance, in a flowing medium flowing at a flow velocity in a conduit of the type wherein a sample of the medium is with drawing from the conduit and is passed through a sensor at a velocity which is lower than the flow velocity, for detecting the variation.
2. Description of the Prior Art
The long response times of gas sensors is a major disadvantage in many applications. Sensor sluggishness can lead to distorted results in e.g. the measurement of variation, such as rapid changes in the concentration of a substance in a flowing medium. Thus, a sluggish sensor would cause change exhibiting a steep pulse slope to appear as if it had a more gradual slope. In the ventilator and anaesthesia field, such slow response time is a particular problem with oxygen sensors, but is also a problem with carbon dioxide sensors.
In an effort to avoid problems associated with sluggish sensors, special measurement methods of varying complexity must often be used. For example, European Application 0 392 503 describes a special technique for determining oxygen consumption and carbon dioxide output from breath to breath in a patient on a ventilator.
Another special measurement method is disclosed in U.S. Pat. No. 4,202,352, wherein exhaled gas from several breaths are stored in a gas storage tube. During analysis, the stored gas is passed through a gas analyzer at a reduced velocity (compared with the velocity of the exhaled gas).
One drawback with this method is the risk of diffusion of gas within the stored gas. In particular, the diffusion will be greatest within portions exhibiting rapid changes in concentration. There is also a risk that the gas analyzer tube itself will cause contamination of the gas. This happens because gas particles near the wall of the tube can have other flow characteristics than gas particles in the middle of the tube (e.g. laminar and turbulent flows have different characteristics).